<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<script type="text/javascript">
		    // 只是声明变量，没有赋值的话，现在的值就是undefined
			var num;
			console.log(num);// undefined
			console.log(typeof(num));// undefined
			num = 3.14;
			console.log(num);// 3.14
			console.log(typeof(num));// number
			num = 'js';
			console.log(num);// js
			console.log(typeof(num));// string
			num = true;
			console.log(num);// true
			console.log(typeof(num));// boolean
			num = null;
			console.log(num);// null
			console.log(typeof(num));// object
			
			
			
			var v1 = 10;
			var v2 = '10';
			if (v1 == v2) { // == 只判断里面的值是不是相等
				console.log('v1==v2');
			}
			if (v1 === v2) {// === 既判断值是不是相等，又判断类型是不是相等
				console.log('v1===v2');
			}
			
			var str = "Hello jS!";
			console.log(str.toLowerCase());//hello js!
			console.log(str.toUpperCase());//HELLO JS！
			console.log(str.charAt(6));//j
		</script>
	</head>
	<body>
	</body>
</html>
